An ion cyclotron uses a fixed magnetic field to deflect an ion moving at some velocity through the field. For a spatially uniform magnetic field having a flux density B, a moving ion of mass m and charge q will be bent into a circular path in a plane perpendicular to the magnetic field at an angular frequency .omega..sub.o in accordance with: .omega..sub.o =qB/m. Thus, if the magnetic field strength is known, by measuring the ion cyclotron frequency it is possible in principle to determine the ionic mass-to-charge ratio m/q. In effect, the static magnetic field converts ionic mass into a frequency analog. Because the cyclotron frequencies for singly charged ions (12.ltoreq.m/q.ltoreq.5000) in a magnetic field of about 3 Tesla span a radio frequency range (10 kHz.ltoreq.f.ltoreq.4 MHz) within which frequency can be measured with high precision, the ion cyclotron is potentially capable of offering extremely high mass resolution and accuracy.
In an ion cyclotron cell, the ions may be formed by irradiation of a neutral gas, solid, or liquid by various known techniques, including the application of electron, ion, or laser beams. The ions are trapped in the cell because the magnetic field constrains the ions to a circular orbit in a plane perpendicular to the field, and a small DC potential is applied to the trapping plates of the cell prevents the ions from escaping in a direction parallel to the magnetic field. However, even ions having the same mass-to-charge ratio and the same initial velocity are created at random points in time, and therefore with random phase, i.e., having random angular positions in their circular paths. These incoherently moving ions cannot produce a detectable signal in the cell. To detect the ions, it is necessary to apply an oscillating electric field in a direction normal to the magnetic field and at the ions natural cyclotron frequency to drive the ions to orbit coherently in a larger radius orbit.
Various techniques have been used to detect the resonant ion cyclotron motion. One technique, as used in the omegatron type ion cyclotron resonance mass spectrometer, measures the current produced as ions continuously spiral outward into a detector plate. Another technique measures the power absorbed by the resonant ions from the exciting electric field. Such techniques generally rely on excitation of the ions with an oscillating electric field at a single frequency to detect ions of a single mass at a time. To collect a spectrum over a range of masses, either the excitation frequency or the magnetic field must be slowly swept. Both of these detection techniques were found to be badly limited with respect to mass resolution, sensitivity, and the time required to gather a mass spectrum. Significant increases in resolution, sensitivity, and speed have been obtained using Fourier transform techniques wherein the whole spectrum is excited at once and the whole spectrum is thereafter detected at once. Such Fourier transform ion cyclotron resonance spectroscopy techniques are described further in U.S. Pat. No. 3,937,955 issued to Comisarow et al., the disclosure of which is incorporated herein by reference.
Since introduction of Fourier transform mass spectrometry (FTMS), significant progress has been made in improving the detection of the resonant ions--for example: by reducing the base pressure in the cells, use of superconducting solenoid magnets, extending the bandwidth of the detection electronics, shielding of the transmitter and detector leads and using a differentially pumped dual cell or external ion source. However, the extremely high resolution (selectivity) realized by FTMS for ion detection cannot be achieved for ion excitation with current art techniques. This has proved a severe restriction for several experiments such as collisionally activated dissociation (CAD), for which FTMS is otherwise ideally suited, and which is of great importance to mass spectroscopists. What is required of an ion excitation technique for FTMS is the ability to selectively excite ions of arbitrary mass-to-charge ratio (hereafter denoted m/z) to arbitrary radii while not exciting other ions present. If the ion excitation is for the purpose of subsequent ion detection, it is necessary to know the ion orbital radius in order to quantify the number of ions from the magnitude of the detected signal. If the ion excitation is for the purpose of subsequent collision with target molecules or ions, it is necessary to achieve a desired ion orbital radius to achieve a desired ion kinetic energy.
Various ion excitation methods are in use today or have been proposed for use in FTMS. The simplest is burst excite, which is a fixed frequency, fixed amplitude sinusoidal signal applied to the cell excite plate for a fixed time. This excitation signal has the familiar (sin x)/x shape in its frequency domain magnitude spectrum. It is possible, using burst excite, to excite ions of one m/z to a desired orbital radius while not exciting at all ions of a second m/z. However, the only adjustable parameters are the sinusoidal frequency, amplitude, and duration, so the excite amplitude spectrum can only have a (sin x)/x shape, which is not suitable when ions of many different m/z are present.
An extension of the burst excite technique is to gradually sweep the frequency of the sinusoid from one frequency to another to excite all ions whose cyclotron frequencies are in that range. This is called sweep (also chirp) excite and is described in the previously mentioned Comisarow, et al. patent. Most commercially available FTMS instruments today use this method. Because this is a frequency modulated signal, the shape of its amplitude spectrum is not available as a convenient closed form equation. The spectral shape is generally a single band with relatively uniform amplitude at the band center, amplitude ripples which are worst at the band edges, and a gradual decrease in ripple amplitude towards zero outside the band. Both the intensity and location of the ripples as well as the sharpness of the band edges depend on the sweep parameters (sweep rate, start and stop frequencies) in such a manner that arbitrarily sharp band edges and low ripple cannot be achieved at the same time. In addition, sweep excitation necessarily excites all ions with resonant frequencies between the sweep start and stop frequencies and thereby does not allow selective excitation of ions with only certain ranges of m/z values. Such broad band excitations also cannot be used to eject ions of all but one or a few selected m/z values.
Another ion excitation method for FTMS is based on sinusoidal bursts and may be denoted pulse sequence excitation. A sequence of sinusoidal bursts is constructed with the frequency, phase, and starting time of each burst such that the amplitude spectrum of the sequence approximates the desired excite amplitude spectrum. High selectivity is possible for simple spectral shapes, but it is difficult to construct pulse sequences to approximate arbitrary excite spectra.
Impulse excitation consists of a single narrow pulse. This method is broadband only, so no selectivity is possible. Also, very high voltages are required to deliver sufficient energy to the ions, due to the short time duration of the pulse. Pseudo-random noise excitation uses a white noise sequence to excite ions over a wide mass range. No selectivity is possible with this method either, but much lower voltages are required than for impulse excitation.
An improved technique for tailoring the excite amplitude spectrum to excite ions of particular m/z values is set forth in U.S. Pat. No. 4,761,545 to Marshall et al., entitled Tailored Excitation For Trapped Ion Mass Spectrometry, the disclosure of which is incorporated herein by reference. This method, which may be denoted as stored waveform inverse Fourier transform excitation, takes an arbitrary excitation amplitude spectrum and inverse Fourier transforms it to give a time domain waveform. This waveform is then used as the excitation signal. There are two inherent problems with this method.
The first difficulty is that the resulting time domain waveform has a very high peak-to-average power ratio, particularly when the starting excite amplitude spectrum is broadband. This requires the use of power amplifiers with impractically large output voltages to achieve adequate ion orbital radii. U.S. Pat. No. 4,761,545 uses phase scrambling to overcome this problem. A phase is assigned to each frequency in the starting excite amplitude spectrum such that a smaller number of frequencies are in phase at any point in the resulting time domain waveform. However, phase scrambling distorts the excite amplitude spectrum such that it is not possible to achieve arbitrary excite spectra and suitably low peak excitation voltages at the same time.
The second inherent problem with the stored waveform technique is that if there exist any discontinuities in the starting excite spectrum or in any order derivative of this spectrum, truncating the resulting time domain waveform to finite length introduces Gibbs oscillations into the corresponding excite amplitude spectrum. These amplitude fluctuations can be quite large and limit the excite selectivity to an unacceptable level. This problem is frequently encountered in the field of digital signal processing, and the accepted solution is to truncate the time domain waveform gradually on both ends by multiplying it by a window function (apodizing function). A window function has a value of zero at both ends, a value of one in the center and varies smoothly in between. This removes the Gibbs oscillations, but if applied to a stored waveform which has been phase scrambled, it can cause severe distortion of the excite spectrum. Thus, windowing and phase scrambling, as described above, often cannot be used concomitantly.